Liquid nitrogen cooled beryllium superconductor



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United States Patent 3,301,937 LIQUID NITROGEN COOLED BERYLLIUM SUPERCONDUCTOR Pierre Burnier, Versailles, and Jacques Bonmarin, La

Motte Servolex, France, assignors to Pechiney, Compagnie de Produits Chimiques et Electrometallurgiques, Paris, France Filed Nov. 2, 1964, Ser. No. 408,276 Claims priority, application France, Nov. 8, 1963, 953,119 8 Claims. (Cl. 174-15) This invention relates to coils, conductors and connectors formed of beryllium and cooled to a temperature below 150 K. for use in electrical devices intended for the production of electrical, magnetic or mechanical energy or for the conversion of electrical energy.

It is known that'the weight and bulk of electrical machinery can be materially reduced and their efiiciencies increased by the use of conductors maintained at very low temperature. Such improvements result from the lowering of the resistivity of the conductors at such low temperatures.

The resistivity of the metal is the sum of two independ-' ent terms; the ideal resistivity which corresponds to the inter-action of electrons with the thermal vibrations of the crystalline structure, and the residual resistivity due to the presence of impurities and faults in the structure of the crystalline network. The ideal resistivity can be reduced by reductions in temperature whereas the residual resistivity is, in principle, independent of temperature.

In order to obtain a sutficiently low ideal resistivity, it has been the practice to make use of very low temperatures such as by the use of liquid hydrogen which boils at 204 K. or of liquid neon which has a boiling point of 273 K.

In order to obtain sufficiently low residual resistivity, it has been the practice to make use of conductors having a very high degree of purity. Generally the amounts of impurities are limited to between 1 million.

To the present, the cold-conductor concept has been applied to conductors formed of sodium, copper, or alumium having a high degree of purity and cooled to a temperature of about 20 K. in order to take advantage of the exceptional conductivity of these metals at very low temperatures.

It is an object of this invention to produce electrical devices which make use of cold conductors which are an improvement over conductors of the type heretofore employed and it is a related object to produce cold conductors of beryllium and to provide electrical machinery adapted to embody same.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration, but not of limitation, embodiments of the invention are shown in the accompanying drawings, in which:

FIG. 1 is a graph which relates resistivity P, expressed in microohms-cm., versus absolute temperature for various metals;

FIG. 2 is a graph representing the value of the ratio P/P as a function of the absolute temperature for various metals;

FIG. 3 is a cross-sectional elevational view of an assembly embodying a conductor in accordance with the prac' tice of this invention;

FIG. 4 is a cross-sectional view of a stator for an alternating current rotary machine embodying the features of this invention;

FIG. 5 is a cross-sectional view of a coil or winding in a transformer or circuit breaker embodying the features of this invention; and

and 100 parts per 3,301,937 Patented Jan. 31, 1967 "Ice FIG. 6 is a diagrammatic illustration of an installation for the use of a cold conductor embodying the features of this invention.

In our researches for improvement in the techniques of cold conductors, we have found that beryllium can be used as a cold conductor in a manner to provide for marked improvements in electrical machinery which makes use of cold conductors. Such improvements result from the ability to make use of industrial grades of beryllium and operation of the conductors formed thereof at temperatures even higher than those of liquid hydrogen or neon.' This result is entirely unexpected because beryllium of industrial grade is not particularly pure, and, as shown in FIG. 1, the lowering of the resistivity at very low temperatures is much lesser with beryllium than in the case of copper or aluminum.

In FIG. 1, which relates the fluctuation in resistivity p, expressed in microohms-cm., for various metals, as a function of absolute temperature, it can be seen that the resistivity of extra pure aluminum, copper and sodium drops in proportion to the temperature from ordinary levels to that of 20 K., for example, which corresponds to the boiling point of hydrogen under atmospheric pressure.

The determination of minimum resistivity of the con ductor is not adequate to give an exact idea of the gain in power obtained by machines which make use of cooled conductors. This power gain is partially offset by the necessary expenditure of power for operating the refrigerator, the efficiency of which diminishes as the working temperature is lowered. The refrigerant receives the heat produced by Joules effect in the winding at a temperature T Kelvin, and disperses the heat at a temperature close to the ambient temperature, i.e. approximately 300 K.

An ideal machine working under these conditions would, according to the laws of thermodynamics, have a maximum energy output of T 300-7.

Indeed, an actual machine would have an efficiency less than that of the ideal machine, so that the energy output under consideration would be l/M T/300-T, in which M is a factor which becomes larger as T becomes smaller. For practical purposes, between and 4 K, the value of M is given by the equation:

Taking these formulae into account, the real output of a liquid nitrogen refrigerator operating at 77 K. is 0.135 while that of a liquid hydrogen refrigerator working at 20 K. is 0.02 and that of a liquid helium refrigerator working at 4 K. is only 0.0015.

It becomes possible to determine, as a function of the temperature T of the cooled conductor, the total power which will be expended, on the one hand, by a Joules effect, and, on the other hand, by a refrigerator which will disperse the heat released into the ambient temperature. It is possible to write an equation which is the usual running temperature of electrical appliances.

So long as the ratio P/P remains greater than or close to unity, there is no interest in using the cooled conductor, which would not permit of any real power gain in comparison with a conventional machine with a copper winding.

From FIG. 2, it can be seen that; in accordance with the practice of this invention, beryllium of industrial grade offers a ratio of P/P of the order of 0.5 at the temperature of 150 K. and of the order of 0.15 between 6080 K., whereas aluminum, for example, containing only 40 parts-per million of impurities, does not reach a ratio of the same magnitude until approximately 25 K. and offers nothing of interest at approximately 80 K.

, In the determinations illustrated in FIGS. 1 and 2 use is made of an industrial grade of beryllium having impurities present in an amount greater than 0.1% by weight. It contains approximately 1,000 parts per million of beryllium oxide, 90 parts per million of iron, 25 parts per million of aluminum, 2.0 parts per million of silicon, 1(l parts'per million of nickel, 10 parts per million of chromium and 5 parts per million of manganese.

One of the important advantages in the use of beryllium as a conductor, in accordance with the practice of this invention, is that it becomes no longer necessary to refrigerate the conductor to temperatures as low as that of liquid hydrogen or neon. Minimum power loss is obtained atapproximately the boiling point temperature of liquid nitrogen at atmospheric pressure. In calculating the economy of the method, it is not only a question of minimum efliciency loss which, of course, is of interest; the investments necessary to effect and to maintain the refrigerating equipment must also be considered. Liquefying plants for the production of liquid neon, hydrogen or helium are of high cost. On the other hand, liquid nitrogen is relatively inexpensively produced from liquefaction of air and liquid nitrogen in large quantities is currently available at relatively low cost as a by-product in the liquefaction of air to provide liquid oxygen used in the production of steel by oxygen converters. Liquefied nitrogen offers many other advantages over other liquefied gases or refrigerants in that it is relatively low in cost and readily available; it is non-toxic; it is non-corrosive to the conductors, and it is an excellent electrical insulating material. Thermal insulation is much easier to achieve and maintain at the temperatures of liquid nitrogen than at the temperatures of liquid neon or liquid hydrogen.

Although the use of liquid nitrogen at atmospheric pressure is preferred as the refrigerant for use in the practice of this invention, other liquid or gaseous refrigerants can be used at pressure above or below atmospheric pressure for cooling beryllium conductors.

The joint use of industrial grades of beryllium as a conductor and liquid nitrogen as the refrigerant fluid for low temperature electrical appliances represents a unique combination which cannot be equalled from the technical or economical standpoint by any other pairing of metals and cryogenic liquids.

It will be understood, however, that the results can be still further improved by making use of beryllium of greater purity. With beryllium containing up to 100 parts per million of beryllium oxide and 100 parts by million of other impurities, the power loss at 77 K. drops to approximately 8% of that observed in a copper winding operating at 70 K. and even better results can be obtained by working with beryllium at'40 K. where the ratio of P/P is in the vicinity of.0.'0l5. I a

The following examples, which are given by way of illustration but not by way oflimitation, show certain arrangements of coil-winding machines, conductors or connections fabricated of beryllium and cooled with liquid nitrogen. It is possible, without departing from the scope of the invention, to adopt other methods of cooling or of heat-insulating the appliances.

Example 1 In FIG. 3, illustration is made of a conductor 1 fabricated of beryllium and contained in a heat-insulating sheath. The conductor is intended for the continuous production of magnetic energy to energize a large synchronous machine. I Since the conductor 1 has direct current passing through it, it can be solid, i.e. non-foliated. A longitudinal duct 2 extends through the conductor 1 and liquid nitrogen is circulated through the duct at a pressure within the range of 0.2 to 10 atmospheres absolute. Thermal insulation 3, such 'as aluminized polyethylene-terephthalate reflector screens, is arranged in the space between the two fluid-tight spacedwalls 5 and 6 which is maintained under a vacuum of l0- Torr. Wall 5 is housed in the recess 4 in the machine.

Example 2 In FIG. 4, illustration is made of an alternating current rotary machine embodying the features of the present invention. An alternating current is passed through the conductor 17 and the conductor is placed in an alternating magnetic field. In this modification, the conductor is formed of a stack of thin sheets of beryllium in order to obviate the unfavorable influence of Foucault currents and of the pellicular effect. The conductor could also be made of fine berylium wires insulated one from the other and formed into a cable. Liquid nitrogen is circulated as a refrigerant through conduits 12 arranged externally of the conductor. This assembly is housed within a sheath 16 of insulating or low conductivity material'that is impermeable to liquid nitrogen. The space between the inner sheath 16 and the outer sheath 15, the latter of which is made of similar material, is provided with heat-insulat ing material 13, such as reflector screens, and a very high vacuum is maintained in the space between the two sheaths.

Example 3 In FIG. 5, illustration is made of a winding 21 consisting of thin sheets of fine wires of beryllium which are insulated one from the other and immersed in liquid nitrogen 22. The winding can be used as a winding of a'transformer to produce a magnetic field or ,as the coils of a circuit breaker for converting alternating electrical energy. In such installations, the liquid nitrogen 22 functions as the refrigerating medium and as a dielectric medium since it embodies the dielectric properties required for such an application. The assembly ishoused in a double-walled vessel formed of walls 25 and 26 to define a sealed space provided with thermal insulation 23 and maintained under high vacuum.

Example 4 In FIG. 6, illustration is made of an installation produced to embody the features of this invention and capable of being used for application of the devices described in the preceding examples. A refrigerant apparatus 7 is adapted to feed the electrical appliance 8 with liquid nitrogen flowing through the heat-insulated interconnecting ducts 9. The efl luent from the appliance 8, which may be in the form of liquid nitrogen and/ or vapors thereof, is collected in duct 10 for return 'to the inlet of the refrigerant apparatus 7. A pump 11 maintains the desired vacuum in the insulated spaces of the refrigerator 7, the electrical appliance 8 and the ducts 9 and 10.

It will be apparent from the foregoing that we have provided a marked improvement in the construction and use of cooled conductors, coils and connections for use in electrical apparatus.

It will be understood that changes may be made in the details of construction, arrangement and operation, with out departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. In an electrical device for the production of electrical, magnetic or mechanical energy or for the conversion of electrical energy, a conductor formed of beryllium cooled to a temperature below 150 K. and a liquid refrigerant circulated in heat exchange relationship with the conductor for cooling the beryllium conductor to a low temperature below 150 K.

2. In an electrical device which makes use of a cooled conductor, the improvement which comprises a conductor formed of beryllium, a fluid impervious wall surrounding the conductor, a liquefied gas circulated in heat exchange relationship with the conductor within the fluid impervious wall, another wall spaced from the said fluid impervious wall to define an open space in between, a thermal insulating material in the space between the walls, and in which the beryllium conductor is cooled to a temperature within the range of 40150 K.

3. An electrical device as claimed in claim 2 in which the beryllium conductor is cooled to a temperature within the range of 6580 K.

4. An electrical device as claimed in claim 2 in which the liquid refrigerant is liquid nitrogen.

5. An electrical device as claimed in claim 4 in which the liquid nitrogen is maintained at a pressure of 0.2 to 10 atmospheres.

6. An electrical device as claimed in claim 1 in which the beryllium conductor is cooled to a temperature within the rangeof 4()1SO K.

7. An electrical device as claimed in claim 1 in which the beryllium conductor is cooled to a temperature within the range of 65-80 K.

8. An electrical device as claimed in claim 1 in which the beryllium conductor is cooled with liquid nitrogen maintained at a pressure within the range of 0.2 to 10 atmospheres.

No references cited.

LARAMIE E. ASKIN, Primary Examiner. I. F. RUGGIERO, A ssislant Examiner. 

1. IN AN ELECTRICAL DEVICE FOR THE PRODUCTION OF ELECTRICAL, MAGNETIC OR FOR THE CONVERSION OF ELECTRICAL ENERGY, A CONDUCTOR FORMED OF BERYLLIUM COOLED TO A TEMPERATURE BELOW 150%K, AND A LIQUID REFRIGERANT CIRCULATED IN HEAT EXCHANGE RELATIONSHIP WITH THE CONDUCTOR FOR COLLING THE BERYLLIUM CONDUCTOR TO A LOW TEMPERATURE BELOW 150% K. 